Code division multiple access (CDMA) systems are interference-limited systems. Since all mobile stations operate at the same frequency, internal interference generated within the system plays a critical role in determining system capacity and voice quality. The transmit power from each mobile station is controlled to limit interference while maintaining desired performance objectives, e.g., bit error rate (BER), frame error rate (FER), capacity, dropped-call rate, coverage, etc.
One technique used in CDMA systems to reduce interference is power control. Power control is used on the reverse link in CDMA systems to control the power of signals received at each base station from the mobile stations. The purpose of power control in conventional CDMA systems is to ensure that each mobile station served by a particular base station approximately provides the minimal required signal strength required to ensure its Quality of Service (QoS). In conventional CDMA systems, system capacity is maximized if the transmit power level of each mobile station is controlled so that its signals arrive at the base station receiver with the minimum required signal-to-noise ratio (SNR). In third generation (3G) CDMA systems that allow variable data rates, the received power of mobile stations operating at the same data rate should be the same, although the received power of mobile stations operating at different data rates may be different because higher data rates require higher transmission power to maintain signal quality standards.
As a mobile station moves within the network, the channel conditions change continuously due to fast and slow fading, shadowing, number of users, external interference, and other factors. Power control algorithms dynamically control the transmit power on the reverse link to maintain a minimum required SNR at the base station under all conditions. Both open loop and closed loop power control are used for power control on the reverse link. In open loop control, the mobile station monitors the received signal strength on the forward link and varies its transmit power inversely in proportion to the measured signal strength. However, fading sources in mobile radio systems require much faster power control than is possible with open loop control. Fast power control may therefore be provided by the closed loop power control mechanism. In closed loop power control, the base station measures the strength of the pilot signal from the mobile station, computes the SNR, and sends power control commands to the mobile station directing the mobile station to either increase or decrease its transmit power depending on the received SNR. The power control commands typically comprise power control bits (PCBs), which are sent at a rate of 800 bps. A bit value of “1” directs the mobile station to decrease its transmit power. A bit value of “0” directs the mobile station to increase its transmit power. In conventional CDMA systems, the traffic channels have a fixed power offset from the pilot channel, so controlling the power of the pilot channel effectively controls the transmit power of the traffic channels as well.
Revision D of the cdma2000 standard includes a high-speed reverse link traffic channel. Mobile stations may transmit at a variable rate on the reverse link traffic channel. The cdma2000 standard specifies the rates at which the mobile station may transmit. Associated with each rate level is a pilot reference level and power offset that varies depending on the data transmission rate to ensure that the received pilot strength is sufficient to support that data transmission rate. As described above, the inner loop power control mechanism for cdma2000 operates by maintaining the received SNR of the reverse pilot channel at some predetermined level, referred to herein as the power control set point. Thus, when the mobile station changes its data transmission rate, the base station needs to adjust the power control set point.
In systems where the base station controls and schedules the data transmission rate of the mobile station, the base station can account for the different pilot reference levels to perform inner loop power control. In this case, the mobile station negotiates with the base station so that the base station can select the appropriate data transmission rate for the mobile station based on the mobile station's desired data transmission rate and the reverse link load. Allowing the mobile station to autonomously select its data transmission rate on the reverse traffic channel would avoid the negotiation process between the base station and the mobile station. However, if the mobile station autonomously changes its data transmission rate such that a new pilot reference level is transmitted by the mobile station, the sudden change in the received strength of the pilot signal at the base station would disrupt the inner loop power control mechanism unless the base station was notified of the rate change in advance.
It has been proposed that the mobile station transmit a rate indication to the base station before it changes its data transmission rate so that the base station can account for differences in pilot signal levels at different rates. Such a proposal includes a complicated signaling scheme to ensure that the base station detects the rate indication before the mobile station transmits data frames at the new data transmission rate. However, the signaling required results in significant delays between the time that the mobile station selects a data transmission rate and the time that it is allowed to transmit at the selected rate. Given that channel conditions and the availability of communication resources change with time, the mobile station needs to select a data transmission rate conservatively to ensure that this system has sufficient communication resources when it starts transmitting at the selected rate. The need to be conservative in rate selection means that the system is not functioning as efficiently as possible.